Polyvinyl alcohol (PVOH) films are often used in the art to form unit dose packages, as the films exhibit good strength, impact resistance, and are soluble in water. The solubility of PVOH films, however, rapidly decreases when exposed to certain chemicals, such as oxidizing chemicals, acid chemicals, alkali chemicals, chlorine-containing substances, salts with polyvalent metals, boric acid, polyamines, insecticides, herbicides, among others. Thus, in applications which employ these types of chemicals, the use of PVOH packaging is restricted, as the effective shelf-life of the unit dose products is limited by their solubility in water.
Modified PVOH films, having co-monomers and/or various additives, have been shown to improve the chemical resistance of the films. For example, see U.S. Pat. Nos. 6,608,121, 6,166,117, 6,787,512, 6,821,590, 7,005,168, and 7,745,517, among others, disclosing use of various plasticizers, additives, and co-monomers such as N-vinylamide, carboxyl and carboxylate functional groups, sulfonic acid functional groups.
U.S. Pat. No. 5,102,950, assigned to Kuraray Co., Ltd., discloses a PVOH film formed from a copolymer consisting of vinyl alcohol units, vinyl ester units, and copolymerization units containing a 2-pyrrolidone ring. Other comonomers are also contemplated, including sulfonic acid groups, quaternary ammonium structures, and others. For use in dose packaging films, it is disclosed to use plasticizers such as a polyhydric alcohol and linear sugaralcohos having 4 to 6 carbon atoms.
U.S. Pat. No. 6,956,070 and EP1251147, also assigned to Kuraray Co., Ltd., teach that films containing a 2-pyrrolidone ring have a problem of insufficient cold water solubility. Instead, it is disclosed that it is possible to provide a water-soluble film simultaneously satisfying the requirements in regard to water solubility, biodegradability, and physical properties when the PVOH is modified with monomers including an N-vinylamide, a carboxyl group and lactone ring.
U.S. Pat. No. 6,166,117, also to Kuraray Co., Ltd., discloses a water-soluble film including a sulfonic acid group modified polyvinyl alcohol in admixture with gallic acid. In this patent, it is noted that 2-acrylamido-2-methylpropanesulfonate-modified polyvinyl alcohols are unsuitable for storage of acidic chemicals for long periods of time, as they lose their function as a water-soluble film. The gallic acid is taught as necessary to achieve the desired properties.